It is known that colloidal silica is added to water-based resin coating compositions for the purpose of modifying or improving the performance of films formed from the water-based resin coating compositions which are attracting attention from the standpoints of preventing environmental pollution and ensuring safe and hygienic working atmospheres. However, there is a disadvantage that the bonding between the colloidal silica particles added and the organic polymer emulsion is weak and, as a result, the resulting coating films have poor durability. It has therefore been extensively attempted in the field of coating compositions to improve the interfacial adhesion by grafting an organic polymer onto colloidal silica particles.
For example, JP-B-62-58630 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a surface-treating agent for metals which is obtained by adding a colloidal silica and a chromium compound to an acrylic resin obtained by the multi-stage emulsion polymerization of an organosilicon compound having both a reactive unsaturated group and an alkoxy group with acrylic monomers. The surface-treating agent gives coatings which impart good corrosion resistance and coating suitability to the metal surface and have excellent adhesion, deep draw processing properties, and anti-block properties.
This surface-treating agent, however, has the following disadvantage. During the process for producing the acrylic resin, alkoxy groups are hydrolyzed into silanol groups, which in some cases form siloxane bonds (--SiOSi--) through subsequent condensation reactions. That is, the siloxane bonds are unevenly distributed in the molecules of the acrylic resin produced, although it is desirable that the portions which can exert an interaction with colloidal silica particles added be evenly distributed in the acrylic resin to improve the interfacial adhesion between the silica particles and the resin. The uneven distribution of siloxane bonds in the molecules results in that siloxane bonds do not present over the whole interfaces between the acrylic resin and colloidal silica particles added. Therefore, sufficient interfacial adhesion cannot be obtained.
JP-A-59-71316 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses an aqueous resin dispersion which is obtained by emulsion-copolymerizing an acrylic monomer, a styrenic monomer, and an organosilicon compound having both a reactive unsaturated group and an alkoxy group in an aqueous medium in the presence of a colloidal silica. The aqueous resin dispersion can form films having excellent durability and good resistance to stains.
JP-A-61-155474 discloses an aqueous coating composition in which an aqueous resin dispersion obtained by emulsion-polymerizing an acrylic monomer and an organosilicon compound having both a reactive unsaturated group and an alkoxy group in the presence of a colloidal silica and a water-soluble or water-dispersible acrylic copolymer having an alkoxysilyl group are contained as binder components. The aqueous resin dispersion gives films having durability, flame retardant properties, resistance to stains, dew condensation-preventing properties, and other properties.
However, both of the above two proposals have the following disadvantage. Since in either case the emulsion copolymerization of an organosilicon compound with organic monomers is conducted in the presence of a colloidal silica, the radical copolymerization at olefin moieties, the condensation reaction of alkoxysilyl groups with Si--OH groups on the surfaces of colloidal silica particles, and the condensation reaction among alkoxysilyl groups proceed simultaneously. For this reason, it is difficult to complete the desired condensation reaction of alkoxysilyl groups with Si--OH groups on the surfaces of silica particles. Further, part of the colloidal silica particles do not take part in this condensation reaction. Hence, the interfacial adhesion between the organic polymer and the colloidal silica particles cannot be increased to a sufficient level. It may, therefore, be thought that the above two proposals can be made more effective if the emulsion polymerization is conducted in two stages in a manner such that siloxy groups containing a highly radical-copolymerizable, reactive unsaturated group are first incorporated into the surfaces of colloidal silica particles, and then an organic monomer is copolymerized therewith. However, such a process has not been studied to present.
Recently, as one approach to develop highly functional polymer materials, modification of organic polymers with silicones is attempted to impart to the organic polymers the heat resistance, cold resistance, weathering resistance, flame retardant properties, and other excellent properties possessed by the silicones. However, since the silicone is a specific polymer having a siloxane skeleton and, hence, has poor compatibility with ordinary polymers having carbon skeleton, compositions obtained by merely blending silicones with ordinary organic polymers cannot satisfactorily retain the desired properties of the silicones. It is, therefore, necessary to chemically bond a siloxane skeleton to a carbon skeleton by grafting or other means.
As such a technique, polymerization of a vinyl monomer in the presence of a polyorganosiloxane containing a vinyl or allyl group is disclosed in, for example, JP-A-50-109282. This polymerization gives a graft copolymer, thus imparting improved impact strength to the resulting resin. Furthermore, JP-A-60-252613, JP-A-61-106614, and JP-A-61-136510 propose to obtain a graft copolymer having a high degree of grafting and excellent impact strength by polymerizing a vinyl monomer in an emulsion of a polyorganosiloxane containing an acryl or methacryl group.
However, the graft copolymers obtained by the above methods are defective in that their mechanical properties such as impact resistance have not been improved sufficiently because polyorganosiloxanes alone which do not have sufficient strength are used as the rubber component. For the purpose of improving the mechanical properties of such polyorganosiloxanes, a combined use of a tri- or tetrafunctional crosslinking agent such as an alkyltrialkoxysilane, tetraalkoxysilane, etc., has been attempted to improve the strength, but attainable improvement in strength is limited and a satisfactory polyorganosiloxane has not yet been obtained.
One possible approach to overcome the problems accompanying the attempts to modify or improve ordinary organic resins using silicone or adding a reinforcing material such as colloidal silica is to incorporate a reinforcing material such as silica into domains of a polyorganosiloxane. As a result of the incorporation of such a reinforcing material into polyorganosiloxane domains, not only the mechanical properties of the resulting graft copolymer are improved, but also new properties are expected to be imparted to the graft copolymer. However, in order to realize the above approach, a silicone emulsion in which a polyorganosiloxane and a silica particle are contained in the same micelle is required, and preferably an emulsion of colloidal silica-core silicone-shell particles each of which comprises a core which is a colloidal silica particle and a polyorganosiloxane shell covering the core through siloxane bonds is desired.
Although no literature has so far been found which reports or proposes a process clearly intended for the preparation of the above-described core-shell particles, there are some literature references disclosing processes which may form such core-shell particles. For example, JP-A-61-16929, JP-A-61-271352, and JP-A-61-272264 disclose a process for obtaining an aqueous emulsion of a silicone reinforced with colloidal silica particles, in which hydroxyl-terminated polyorganosiloxane is condensed using a sulfonic acid-type emulsifying agent in the presence of an acid colloidal silica. However, since the polysiloxane used as a raw material has a relatively high degree of polymerization, it is extremely difficult for the initial homogenizing step to include the polysiloxane and a colloidal silica particle in the same micelle and, as a result, the above process necessarily yields an emulsion containing not only the desired core-shell particles but also those colloidal silica particles and polyorganosiloxane which have not participated in the polycondensation. Therefore, even if this method is applied to the above-described process for producing a graft copolymer, it is difficult to exhibit the effect by the addition of colloidal silica.
JP-A-1-234468 discloses a reactive microgel composition which is obtained by mixing an organosilane compound having a polymerizable unsaturated double bond and an alkoxy group and, if necessary, other alkoxysilanes with a silica sol, and then subjecting the resulting mixture to hydrolysis. The reactive microgel composition cures upon exposure to irradiation or heat to give a film having excellent film properties. However, this composition cannot impart to organic polymers the heat resistance, cold resistance, weathering resistance, and other properties possessed by silicone resins, and is not intended for use in such a purpose. The above reference also does not suggest such use of the composition.